Solid-state memory devices have become quite popular due to several advantages over typical active memory devices. First and foremost, the solid-state memory device comprises no moving parts, so it consumes less power and provides robust reliability. Moreover, solid-state memory devices are resistant to mechanical stress, such as impacts and vibration.
One type of memory used for solid-state memory devices is the flash memory device. The flash memory device may be based upon NAND or NOR logic gates, for example. For example, U.S. Patent Application Publication No. 2009/0080236 to Nakamura discloses a memory device. The memory device comprises a plurality of memory cells, and bit lines coupled to each memory cell. The memory device provides a supply voltage to each of the memory cells via the bit lines. Even though solid-state memory devices use less power than typical approaches, there is a desire to reduce the power consumption of solid-state memory devices.
Another approach to solid-state memory is a phase-change memory. These devices, rather than using the floating gate approach of flash memory, use chalcogenide glass for storing data. In particular, chalcogenide glass may be forced to take either amorphous or crystalline form by exposing it to a prescribed level of heat. The current state is detected by detecting the resistivity of the chalcogenide glass, which predictably varies based upon the form thereof.
A potential drawback to these phase-change memories is increased power consumption. In particular, the current required to phase-change the chalcogenide glass, i.e. program the memory cell, may be quite high, thereby increasing power demands. In typical approaches, the chalcogenide glass has been heated and cooled using a titanium nitride (TiN) heating element.
One approach to such a power consumption drawback is disclosed in “Low-Power Switching of Phase-Change Materials with Carbon Nanotube Electrodes,” Pop et al., Science Magazine, Vol. 332, Apr. 29, 2011, the contents of which are hereby incorporated by reference in their entirety. This approach comprises forming a phase-change memory cell using carbon nanotubes as the electrodes. The carbon nanotubes extend horizontally on a substrate, reducing memory density.